As used herein, the term “arrhythmia” refers to any abnormal heart rhythm that may be dangerous to the patient and specifically includes fibrillation, tachycardias, supraventricular tachycardias (SVT), ventricular tachycardias (VT), ventricular fibrillation and flutter (VF), and bradycardia. As further used herein, the term “therapy” refers to any means used by the ICD to restore normal heart rhythm such as defibrillation, cardioversion, antitachycardia pacing (ATP), antibradycardia therapy and drug infusion. The disclosed invention has application to ICDs which treat tachyarrhythmias (abnormally high heart rates).
It has been common practice to monitor the heart rate, or more commonly the ventricular rate, of a patient and classify the cardiac condition of the patient based on this heart rate. For example, tachyarrhythmia may be defined as any rate in a range above a designated threshold VT1. This range is then divided into ventricular tachycardia and ventricular fibrillation (and flutter) zones. The ventricular tachycardia zone may be further divided into slow ventricular tachycardia and fast ventricular tachycardia zones.
However, using heart rate as the sole criterion to classify the cardiac condition of patient is problematic. Many physically active patients have heart rates during exercise that overlap with their tachycardia rates. Other patients exhibit supraventricular tachycardias, the rates of which overlap with rates of tachycardias of ventricular origin. These supraventricular tachycardias are often well tolerated and require no intervention.
When physicians classify an intracardiac rhythm, they typically examine the morphology of the electrocardiogram in addition to the heart rate. The shape of an intracardiac complex holds information on the origin and sequence of the heart's electrical activity. A normal intracardiac complex traverses the AV node and is conducted by specialized cardiac tissue throughout the ventricles. This results in a distinctive complex morphology. A tachycardia of ventricular origin often has a different morphology due to its ectopic origin and conductance through cardiac muscle tissue.
As such, in addition to monitoring heart rate, ICDs are capable of performing morphology discrimination to classify the cardiac condition of the patient. A template based on the morphology of a “known” signal is stored in the ICD. The “known” signal can be, for example, a signal collected during a period where a patient is known to exhibit a normal sinus rhythm. By comparing the morphology characteristics (number, amplitude, sequence and polarity of waveform peaks, as well as the area of the peaks) of an arrhythmia to the template, the ICD can calculate the match between the waveforms. For a complete description of morphology discrimination, refer to U.S. Pat. No. 5,240,009 to Williams, entitled “Medical Device with Morphology Discrimination” and to U.S. Pat. No. 5,779,645 to Olson et al., entitled “System and Method for Waveform Morphology Comparison,” which patents are hereby incorporated by reference.
Once it is determined that a patient suffers from one of these cardiac conditions, the ICD is programmed to provide a corresponding therapy. Typically, ventricular tachycardia is treated with a therapy consisting of low-energy pacing pulses designed to capture the ventricle. This therapy is referred to as AntiTachycardia Pacing therapy (ATP). Ventricular fibrillation, on the other hand, is treated more aggressively with high energy shocks. The ICD is programmed with parameters for various types of therapies and the rates defining the therapy zones corresponding to the respective therapies.
Over the years, the number of programmable parameters has been increasing steadily. A modern ICD has up to 200 or more programmable parameters. A major challenge for both the ICD manufacturer and the clinician is to select proper values for these parameters. While the manufacturer may provide nominal or default values for the parameters, these nominal values may not be proper for all patients and it is up to the clinician to change them using statistical information and his personal experience.
For example, approximately 7% of ICD patients exhibit ventricular tachycardia that is typically too slow to be detected by conventional rate cutoffs. Thus, the ICD may fail to discriminate between normal sinus rhythm and slow ventricular tachycardia. However, changing and adapting the parameters have proven to be difficult and it has been found that most clinicians leave the majority of the parameters at their nominal values.
Furthermore, some of the parameters, including the thresholds defining the antitachycardia therapies described above, should be changed periodically to conform to the changing condition of the patient.
It is, hence, desirable to provide an ICD capable of selecting or adjusting some of its parameters automatically so that the clinician does not have to set them on implantation or adjust them each time the condition of the heart changes. More importantly, it would be advantageous to provide an ICD which can adjust some of its parameters adaptively, quickly and efficiently setting them at their optimal levels, and resetting or re-adjusting them automatically as the condition of the patient changes.
The present invention addresses the problem of automatically and dynamically adjusting the thresholds which define the various tachyarrhythmia therapy zones.